1. Field of the Invention
The present invention relates to a microporous stencil sheet for use as a master for stencil printing, and to a method for making a stencil master using the stencil sheet and a stencil master obtained by the method.
2. Description of the Related Art
Known stencil printing sheets (stencil sheets) used for stencil printing include heat-sensitive stencil sheets that comprise a thermoplastic film and a substrate tissue paper laminated together using an adhesive. These stencil sheets are perforated by irradiation with infrared rays or by the use of a thermal head (a thermal printing head) in order to make a master. As stencil printing ink, a water-in-oil emulsion ink is mainly used.
Stencil printing is effected by perforating a stencil sheet in a printing portion of a desired printed image to make a stencil master, and passing ink from the tissue paper side through the perforated portion of the master onto printing paper. In the following description, a printing portion refers to all parts to be printed of the printing paper. The amount of ink transferred (hereinafter referred to as xe2x80x9cink transfer amountxe2x80x9d) is controlled by, for example, the viscosity of ink, the density of tissue paper (to control the passing resistance of ink), the density of the perforated area of film, printing pressure, and printing pressure time.
In stencil printing, large amounts of ink are transferred to printing paper, requiring considerable time for the ink to permeate the printing paper. Improvement in drying property of such ink is therefore required. Specifically, the ink is resistant against permeating the surface of printing paper, and fingers become dirty when printed matter is touched immediately after printing. Additionally, the ink is transferred to, for example, the rubber rollers of a printing machine and then to printing paper, thereby making the printed matter dirty, if a second or further color is printed in color printing or if the reverse is printed in duplex printing, immediately after the first printing. Due to these problems, once the printing paper is printed in a first step, considerable time is required (e.g., from about 10 to about 20 minutes) before subjecting the work to the subsequent step. On the other hand, if the ink transfer amount is simply reduced, print quality is deteriorated such that the resulting printed image may appear blurred.
As a possible solution to the problem mentioned above, several techniques have been proposed in order to reduce the ink transfer amount without the deterioration of print quality, such as (1) a technique in which the density of tissue paper is increased to increase the passing resistance of ink, (2) a technique in which the element size of a thermal head is decreased to minimize the area perforated in thermal head master-making, and (3) a technique in which a product of printing pressure and printing pressure time is kept to minimum levels.
However, according to technique (1), the number of contact points between the film and tissue paper increases with the increase of tissue paper density, and molten components of the film tend to accumulate at these contact points, thereby deteriorating the perforating property in master-making, resulting in insufficient perforation. Additionally, open pore distribution of such tissue paper is difficult to control due to the manufacturing method of tissue paper; if the paper has a large distribution of open pores, variation in the perforation area can result. Accordingly, the resulting perforated stencil master has portions with high ink-permeability and with low ink-permeability, making the uniform transfer of ink difficult.
According to technique (2), by increasing the packing density of thermal head elements, a density (resolution) of up to 600 dpi is achieved. However, the resulting perforated pores are of sizes ranging from about 20 to about 40 xcexcm, and are still insufficient to control the ink transfer amount, since the pore size must generally be 20 xcexcm or less in order to reduce the ink transfer amount in printing using a low-viscosity ink.
According to technique (3), the mean ink transfer amount can be controlled. However, because the tissue paper has a large distribution of open pores, and the areas perforated by a thermal head vary, uneven amounts of ink are transferred, as described above. Therefore, this technique does not provide a solution to the problem that the drying property of ink is low in portions where large amounts of ink are transferred.
As a possible alternative solution to the above problem, an attempt has been made to decrease the viscosity of ink, to increase the permeability of ink through the printing paper, thereby improving the drying property. However, this is also not a practical solution, since the ink transfer amount increases with the decrease of ink viscosity, necessitating a further decrease in the perforation size of the stencil master, although there is a limit to how far the element density of a thermal head can be increased.
As described above, to date no printing technique has been proposed in stencil printing which can yield quick drying of printed matter while maintaining the quality of the printed image, without the use of a reactive ink such as an UV-curable ink.
Accordingly, an object of the present invention is to provide a microporous stencil sheet that can control the ink transfer amount when a low-viscosity ink with high permeability through printing paper is used, thus improving the quick-drying property of ink in stencil printing. Additionally, a further object of the present invention is to provide a stencil master and a method for making a stencil master using the microporous stencil sheet, that can be used for stencil printing using a low-viscosity ink.
The present inventors found that the use of an inelastic resin film with predetermined air permeability and thickness as a stencil sheet can control the ink transfer amount when a low-viscosity ink is used, and thereby accomplished the present invention.
Specifically, the present invention provides, as an aspect, a microporous stencil sheet composed of an inelastic resin film for use in stencil printing using a low-viscosity ink of a viscosity ranging from 0.001 to 1 Paxc2x7s. This microporous stencil sheet has an air permeability ranging from 1 to 600 seconds and a thickness ranging from 1 to 100 xcexcm. The stencil sheet satisfying such requirements can appropriately control the ink transfer amount of a low-viscosity ink of a viscosity of 0.001 to 1 Paxc2x7s, which is highly permeable through printing paper. Consequently, the drying property of ink in printed matter can be markedly improved as compared with conventional inks (viscosity: 2 to 10 Paxc2x7s). Additionally, the ink transfer amount can be reduced, thereby preventing running of the ink on printed matter. The film constituting the stencil sheet is composed of an inelastic resin, and even when it is pressed against, for example, a thermal head for heat fusion, the film is not deformed, thereby yielding an increased precision in master-making. Furthermore, no deformation of printed images occurs even when stencil printing is performed at a high printing pressure.
In another aspect, the present invention provides a method for making a stencil master using a microporous stencil sheet, in which the microporous stencil sheet includes a thermoplastic resin film and has an air permeability ranging from 1 to 600 seconds and is of a thickness ranging from ito 100 xcexcm. The method includes the step of closing the micropores of the microporous stencil sheet in a non-printing portion of a desired printed image by heat fusion. thereby making it ink-impermeable. By closing the micropores of the non-printing portion in this manner, a stencil master can be made as a mirror image so that ink permeates the micropores of the stencil sheet only in the printing portion.
In a further aspect, the present invention provides a method for making a stencil master using a microporous stencil sheet, in which the microporous stencil sheet has an air permeability ranging from 1 to 600 seconds and is of a thickness ranging from 1 to 100 xcexcm. This method includes the step of closing the micropores of the microporous stencil sheet in a non-printing portion of a desired printed image by deposition of a resin and/or wax, thereby making it ink-impermeable. In this method, the resin and/or wax is preferably deposited from a thermal transfer sheet by fusion transfer, thereby closing the micropores.
The present invention also provides a stencil master obtained by the methods mentioned above.